Alcohol based fuels are one alternative to oil based fuels such as gasoline. Alcohol fuels have a lower energy density as compared to the energy density of gasoline, but an alcohol based fuel can lower the possibility of engine knock since alcohol fuels can decrease cylinder temperatures via a higher heat of vaporization. Consequently, engines that operate with alcohol can make up for at least a portion of the difference in energy density between fuels via operating at conditions that may cause the same engine operating with gasoline to knock. However, engines that operate with higher concentration alcohol fuels may have a more difficult time starting at lower engine temperatures because it can be more difficult to vaporize alcohol in the engine at lower temperatures.
The inventors herein have recognized the above-mentioned disadvantages and have developed a method for starting an engine, comprising: adjusting an effective compression ratio of a cylinder and an amount of residual exhaust gas in the cylinder of the engine in response to a number of combustion events since engine stop and a concentration of alcohol in a fuel supplied to the cylinder during a first engine start; and adjusting the effective compression ratio to a compression ratio less than the compression ratio during the first engine start, and adjusting an amount of residual exhaust gas in the cylinder during the second engine start to less than the amount of residual exhaust gas in the cylinder during the first engine start during a second engine start where a concentration of alcohol in a fuel supplied to the cylinder during the second engine start is less than the concentration of alcohol in the fuel supplied to the cylinder during the first engine start.
By adjusting an effective compression ratio of a cylinder and an amount of residual exhaust gas in the cylinder in response to a number of combustion events since engine stop and a concentration of alcohol in a fuel supplied to the cylinder, it may be possible to provide more reliable engine starting while an engine is operated with higher concentrations of alcohol. For example, increasing a compression ratio of an engine as a concentration of alcohol in a fuel supplied to the engine increases may improve alcohol vaporization and combustion at lower temperatures. In particular, a compression ratio of an engine or cylinder can be set to a relatively high value for at least a first combustion event since engine stop so that engine compression work can be used to heat the fuel and improve fuel vaporization. However, it may not be desirable to keep the engine compression ratio at a high value since doing so may contribute to engine knock. The engine compression ratio may be reduced based on the amount of time the engine is operated, but time may have only a loose relationship to conditions in the cylinder that affect fuel vaporization and knock. Therefore, it may be more desirable and beneficial to decrease the engine compression ratio as a number of engine events increases instead of adjusting the compression ratio solely with respect to time.
The present description may provide several advantages. In particular, the approach may improve engine starting consistency. In addition, the approach may improve engine starting when an engine is started with a higher concentration alcohol fuel. Further, the approach can adjust engine operation for a variety of alcohol concentrations so that the engine may be operated with fuels that have a wide range of alcohol concentration.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.